Telephony devices, such as telephones, analog fax machines, and data modems, have traditionally utilized circuit-switched networks to communicate. With the current state of technology, it is desirable for telephony devices to communicate over the Internet, or other packet-based networks. Heretofore, an integrated system for interfacing various telephony devices over packet-based networks has been difficult due to the different modulation schemes of the telephony devices. Accordingly, it would be advantageous to have an efficient and robust integrated system for the exchange of voice, fax data and modem data between telephony devices and packet-based networks.
Lost-frame recovery mechanisms are known in the art. When a transmitted data element, such as a frame of audio data, does not arrive at its destination by a predetermined playout deadline, lost-frame recovery estimates the characteristics, such as pitch and spectral information, of an audio sample corresponding to the non-arriving frame. Lost-frame recovery produces synthesized digital audio samples corresponding to the estimated non-arriving frame. The estimated audio samples are sent to a decoder state machine, which updates and stores a state of a decoder. The decoder state is used to assist in determining the characteristics of digital audio samples to be played out based on the characteristics of buffered samples that have been previously released to be played out.
In the prior art, if a frame whose characteristics have been estimated as a result of the frame not arriving by the playout deadline is subsequently received, that late frame is discarded, as the contents of the frame have already been estimated and synthesized audio samples have already been output. This results in an estimation error in the decoder state arising from the estimation of the late frame. This error propagates through subsequent states of the decoder state machine.